Oilwell cementing operations are either `primary`, done in the course of drilling a well, or `secondary` (also termed `remedial`), for remedying deficiencies in primary cementing or to alter the well prior to production. Examples of `secondary` cementing processes are `squeeze cementing`, in which a cement slurry is forced through holes or slits in the casing into voids or porous rock formations, and `plug cementing`, during which a small volume of cement slurry is placed in the wellbore to prevent loss of drilling fluid during the drilling phase or to seal off a depleted zone during the production phase.
The basic component of cements in use today is so-called `ordinary Portland cement`, whose raw ingredients are lime, silica, alumina and iron oxide. A pulverised blend of these raw materials is fed into a rotating kiln where temperatures as high as 1500.degree. C. produce a molten mixture; on cooling, one is left with four principle mineral phases: `alite` (tricalcium silicate, abbreviated as C.sub.3 S); `belite` (dicalcium silicate C.sub.2 S); `aluminate` (tricalcium aluminate, C.sub.3 A); and `ferrite` (tetracalcium aluminoferrite, C.sub.4 AF). These four phases are taken from the kiln as a `clinker`, which is subsequently ground with gypsum (calcium sulphate) to produce ordinary Portland cement. The main chemical criterion for classifying Portland cements is the relative proportion of the four principle clinker phases.
Specifications for oilwell cements have been established by the API. There are currently eight classes of API Portland cement, designated Class A through H. These are classified according to the depth to which they are placed, and the temperatures and pressures to which they are exposed. Other physical parameters which appear in the API specification include the fineness of the cement powder and the performance of the cement slurry and set cement under standard tests. The performance tests include measurements of setting time, compressive strength, expansion and free water.
Because of the criticality of the setting time of a cement slurry and because of the complex nature of cement slurry compositions, it is necessary to determine the setting time for a given slurry composition experimentally before the slurry can be used in the field. The conventional method for estimating the setting time of a slurry is the API "thickening time" determined by placing a sample of the slurry in a consistometer in which a bob is rotated at elevated pressure and temperature and to measure the torque required to rotate the bob of the consistometer, the time at which the torque increases to 100 Bearden units being defined as the thickening time.
The thickening of cement is the result of the hydration process and it has been proposed to use ultrasonic measurement to monitor such processes. The article by Stepinsnik, Lukac and Loeuvacu in Ceramic Bulletin, 60, 481 (1981) describes the measurement of the reflection coefficient for ultrasonic shear waves in hydrating cement pastes. The reflected signals depend on several unknowns and must be processed to yield an estimate of the shear modulus. The article by Keating, Hamant and Hibbert in Cement and Concrete Research, Vol 19, pp 554-566, (1989) describes the measurement of the development of a shear modulus in hydrating cement paste by static methods.
The Ultrasonic Cement Analyzer from EG&G Chandler Engineering, Tulsa, Okla. USA, monitors the setting of cements by directly transmitting a compressional ultrasonic signal through a slurry to measure the development of compressional strength of the setting cement. This device also provides an indicator of "thickening time" based on a threshold detection of changes in compressional velocity in the slurry.